class_docstr: Upon initialization, this will create num_triplets triplets based on
  the labels provided in labels_to_indices. This is for experimental purposes,
  to see how algorithms perform when the only ground truth is a set of
  triplets, rather than having explicit labels.
code: def __len__(self):
      return self.fixed_set_of_triplets.shape[0] * 3
  
file_path: pytorch-metric-learning/src/pytorch_metric_learning/samplers/fixed_set_of_triplets.py
imports: import numpy
  import torch
  from torch.utils.data.sampler import Sampler
  from ..utils import common_functions
label_desc: Use image node_id 2 for calling the FixedSetOfTriplets obj's underlying member method code with example usage: obj.__len__() and returns: unknown